You may be onto something Paul, I did notice it bunching up but thought I managed to straighten it out when the window was in, maybe not enough though.
i’m going to go to Clark’s rubber and see if they have something thinner that I can use.

I ended up using Sikaflex as I couldn't trackdown any rubber that was thin enough for the job. I put a genourous bead inside the frame before fitting the window to the frame, then cleaned up the excess.

Yeah, just went to Clark Rubber and they only have the same as what I have which is 1.5mm stuff.
I remember Rares gave me the option of a thin and a thick one so will give them a call tomorry to see what they say. Otherwise, I'll be using windscreen sealer and cleaning up a big mess.

If you go the sealant option Paul ,I would consider the old style sealant ,I wouldn't use Sika just in case you need to remove the frame at any point ,like broken glass or whatever.as Andrew said the silicone spray will help a bit with the rubber.Maybe cut pleats in the rubber where it bends and finish with the sealer.

Well, it has defeated me.
After an hour or so trying to fit it using the silicone spray, all I ended up with is slimy black hands. While the window initially slid in well, it wouldn’t go that last 5% to finish it off.

I'm led to believe that they are installed by using a solvent on the rubber, and even petrol was used in the day as a suitable solvent, to make the rubber soft and slippery, and the solvent evaporation off later letting the window bond to the glass some what.

An old Chevy book I have shows a tool with some timber blocks and some threaded rod used to refit a quarter window glass to a window frame, similar task to what you are doing.

Not sure how thick the Rares Spares seal is but I used 3/64 Glass setting tape (PN: A4263) from CRL + white spirits to soften the rubber. The spirits make the rubber slippery giving you just enough time (not much) to get the frame onto the glass. Also makes a bit of a mess!

Almost 30 years ago a mate had a roll of rubber similar to what you got there & he said that he used petrol to soften it while fitting.
God knows if what you get now is anything like the rubber back then tho!

That could work Paul, I suppose it depends on how thick the tube is though.

I called RS here in rAdelaide and the company that they used to supply them has either gone out of business or stopped making the rubber insert. So, I decided to use mastic sealer. After a bit of stuffing around, I got the window in and it looks quite good too.
I still have to do the passenger side one so I will take a couple of pics while I do it. Hopefully I'll have less sealer on my hands next time.

Glad you got it sorted,I think I'll use the non hardening windscreen sealer for mine when I come to do it.

Which is what "mastic" is, sodaul brand is like $5 a tube roughly mate, Gary. & a friend said Marty or Grant listed that the rubber is obtained from the USA easily before. Please DON'T use a silicone spray if you ever need to paint it, it's virtually impossible to get rid of silicone ever !

So, thought I’d update how I went with using the mastic sealer. Sorry I don’t have any pics.

After I sealed the windows, cleaned up the excess sealer and let them set for a day, I fitted them to my car. With the new RS rubbers in the window, it was very tight, almost too tight to fit the windows, but I managed to work them in. Maybe some of the Rubber Magic before hand would have helped.
Once fitted and with the windows closed, it looked really good and I was pleased with the result...... until I returned the next day and some of the mastic sealer had squeezed out which I had to clean up, which wasn’t so bad if it had only squeezed out on the outside of the window but it did on the inside too which was quite annoying to clean off. Maybe I should have left them for a few extra days before fitting them.
I’ll provide a close up pic of how the sealer looks later on.

All in all, I am happy with how it turned out. If I did it again though, I would use the proper rubber tape that Jeff linked to earlier on.